1(a) Field of the Invention
The present invention relates to an improved apparatus for the decantation treatment of liquid containing therein suspended material, particularly water.
1(b) Brief Description of the Prior Art
French patent no. 1 115 038 relates to an apparatus for the decantation treatment of waste water including a pulsated sludge bed.
French patent no. 2 132 954 (which correspond to Canadian patent no. 991.096 and U.S. Pat. No. 3,788,981) relates to improvements in the apparatus of French patent no. 1 115 038. These improvements lie essentially in the addition of a "finishing member" (i.e. a network of parallel plates or tubes inclined to the horizontal) positioned above the sludge bed (i.e. in a sedimentation zone). This finishing member contributes to stabilize the sludge, to improve the decantation, to "trap" sludge particles that may escape from the bed toward the sedimentation zone and return them to the bed, and make it possible to cope with variations in the quality of the liquid to be treated. This network may be retained above the sludge bed, by steel ropes, supporting platform or other equivalent and known means.
French patent no. 2 196 832 (which correspond to Canadian patent no. 1 024 082 and U.S. Pat. No. 4,156,644) also relates to improvements in the apparatus of French patent no. 1 115 038. These improvements lie essentially in the positioning of a network of parallel plates inclined to the horizontal within the sludge bed and provided with baffles. This network is intended to create concentrated currents of the liquid going upwardly and the sludge settling downwardly in order to increase the coagulation in the sludge bed, the concentration of the sludge bed, the ascentional velocities of the liquid to be treated and the overall efficiency of the apparatus.
Because a sludge bed results of an equilibrium between the amount of particles brought to the sludge bed under a determined flow rate of water, and the amount of sludge withdrawn (i.e. evacuated through an appropriate sludge separator) any sudden change of flow rate of waste water may negatively affect this equilibrium. The density of a fluidized sludge bed varies inversely as the flow rate percolating through it. A sudden decrease of the rise rate through the bed causes compaction and reduction of the volume of the bed. Inversely, a sudden increase of the rise rate causes a dilution of the sludge and expansion of its volume.
Each of aforesaid apparatus shows drawbacks when the upward flow rate of liquid to be treated is suddenly increased. With the apparatus of French patent no. 1,115,038, the upward flow rate is kept low in order to avoid the risk of destroying the sludge cohesion. With the apparatus of French patent no. 2,132,954, the upward flow rate of liquid to be treated may be about double the one used with the apparatus of French patent no. 1,115,038. In fact, slight lacks of cohesion that would normally occur under a higher flow rate of the liquid to be treated, are compensated by the positioning of finishing members above the sludge bed. (i.e. the cohesion of the sludge is stabilized). With the apparatus of French patent no. 2,196,832, when the upward flow rate (which already has a high velocity) suddenly changes to a higher velocity, there is an expansion of the sludge bed that may raise the interface between the agglomerating zone and the sedimentation zone above the network of inclined plates or tubes. When such event occurs the efficiency of the apparatus is negatively affected (e.g. the turbidity of the clarified water is increased).